Radio frequency interference (RFI), e.g., from televisions, transmissions at white space frequencies, satellite downlinks at GPS frequencies, self-interference in transceivers or jamming from an adversary, can cause distortion that degrades or disrupts reception of wireless data signals. Conventional methods for reducing or filtering RFI each have distinct disadvantages.
Analog steering of nulls with array antennas is a spatial domain method that minimizes antenna gain in the direction of an interferer to prevent masking by a high power source of interference. Such antennas are, however, bulky and complex. In addition, they require a steering solution, computation of which consumes significant power and time.
Blanking is a time domain method that excises temporal portions of array signals that contain burst interference as a means of avoiding distortion or masking. While blanking mitigates the need for a steering solution, it leaves the receiver blind in the face of continuous interference.
Another problem associated with wireless communications is self-interference. Self-interference is combated various ways, such as by time domain or frequency domain duplexing to prevent high power transmit signals from entering distortion-prone receiver circuits. In either case, separation of the signals reduces the effective carrying capacity of the wireless spectrum.
Digital filtering methods provide a wide array of tools for isolating signals of interest but require conversion of signals to digital form using distortion-prone active circuits. As a result, analog filtering is used to reduce power before a signal is digitized. Circuits using Type III-IV semiconductor materials such as gallium nitride can tolerate higher power levels before reaching saturation and the distortion that saturation causes, but such materials significantly increase cost and complexity, limiting their use primarily to military applications. Consumer products, by contrast, are quite cost sensitive so they are fabricated primarily with CMOS, a low cost but distortion sensitive material. To compensate for such sensitivity, devices typically are operated at reduced power, which degrades efficiency and link margin in general. Providing inexpensive electronic products that operate free of distortion at higher power than currently possible is clearly desirable.
Another conventional approach is to filter an incoming signal to suppress interference at a particular frequency, typically by using a band-stop or “notch” filter to suppress all signals at the particular frequency. While this approach may remove interference that occurs primarily at a particular frequency, herein referred to as “narrowband” interference, it is not well suited to remove interference that occurs across a broad range of frequencies, herein referred to as “wideband” interference. Yet another conventional approach is to combine a set of narrowband filters to remove wideband interference, but the number of filters required makes this approach relatively costly. Another disadvantage to these approaches is that if the interference is at the same frequency as the desired signal, the band-stop filter not only removes the interference but also removes the desired signal as well, which makes recovery of the desired signal that much more difficult.
Accordingly, in light of these disadvantages associated with conventional approaches to distortion filtering, there exists a need for wideband frequency and bandwidth tunable filtering.